•The significant effect of pore distribution and shape on the effective thermal conductivities of porous media are identified.•Five structural descriptors with explicit physical meanings are ...proposed: shape factor, bottleneck, channel factor, perpendicular nonuniformity, and dominant paths.•These descriptors effectively quantify the anisotropy of pore morphology and strongly correlate with effective thermal conductivities.•The proposed descriptors are incorporated into machine learning models to predict the effective thermal conductivity of porous media and show significantly improved accuracy than using porosity alone.
Understanding the thermal transport mechanism in porous media is important for various engineering and industrial applications. The effective thermal conductivity of porous media is known to be related to the morphology of porous structures. However, existing effective medium approaches usually miss the morphology effects, and numerical simulations are expensive and not physically intuitive. Machine learning methods have recently been successful in predicting effective thermal conductivity, but the lack of descriptors limits physical insights. In this work, we investigate structural features that have significant effects on thermal transport in porous media and identify five physics-based descriptors to characterize the structural features: shape factor, bottleneck, channel factor, perpendicular nonuniformity, and dominant paths. These descriptors can effectively quantify the anisotropy of pore morphology and strongly correlate with effective thermal conductivities. The proposed descriptors are incorporated into machine learning models to predict the effective thermal conductivity of porous media, and the results are shown to be fairly accurate. They provide new insights into the thermal transport mechanisms in complex heterogeneous media.
In this work, the thermoelectric performance of p-type PbS was boosted remarkably through Na doping and introducing Cu2S. Firstly, the electrical transport properties (power factor) of p-type PbS was ...optimized via Na doping, and the maximum ZT ∼ 0.67 was achieved in Pb0.98Na0.02S. Secondly, the total thermal conductivity of Pb0.98Na0.02S was suppressed through introducing Cu2S by means of significantly reducing the electronic thermal conductivity by 80% at 823 K through Cu counter-doping, meanwhile, the power factor of Pb0.98Na0.02S was further enhanced through optimizing carrier concentrations via introducing Cu. The combination of decreasing total thermal conductivities and the optimizing power factors leads to peak ZT ∼1.2 at 823 K in p-type Pb0.98Na0.02S–2%Cu2S. Present results illustrate that introducing Cu2S is an effective method to enhance the thermoelectric performance of p-type PbS through decreasing the electronic thermal conductivity which also has great potential in other thermoelectric materials system.
•Thermoelectric performance of p-type PbS was remarkably boosted by reducing electronic thermal conductivity κele.•κele of p-type Pb0.98Na0.02S was reduced by 80% from ∼0.53 W m−1K−1 to ∼ 0.11 W m−1K−1 at 823 K with introducing Cu2S.•The maximum ZT ∼1.2 at 823 K was realized in p-type Pb0.98Na0.02S–2%Cu2S.
Materials with high thermal conductivity (κ) are of technological importance and fundamental interest. We grew cubic boron nitride (cBN) crystals with controlled abundance of boron isotopes and ...measured κ greater than 1600 watts per meter-kelvin at room temperature in samples with enriched
B or
B. In comparison, we found that the isotope enhancement of κ is considerably lower for boron phosphide and boron arsenide as the identical isotopic mass disorder becomes increasingly invisible to phonons. The ultrahigh κ in conjunction with its wide bandgap (6.2 electron volts) makes cBN a promising material for microelectronics thermal management, high-power electronics, and optoelectronics applications.
Peculiar electron-phonon interaction characteristics underpin the ultrahigh mobility
, electron hydrodynamics
, superconductivity
and superfluidity
observed in graphene heterostructures. The Lorenz ...ratio between the electronic thermal conductivity and the product of the electrical conductivity and temperature provides insight into electron-phonon interactions that is inaccessible to past graphene measurements. Here we show an unusual Lorenz ratio peak in degenerate graphene near 60 kelvin and decreased peak magnitude with increased mobility. When combined with ab initio calculations of the many-body electron-phonon self-energy and analytical models, this experimental observation reveals that broken reflection symmetry in graphene heterostructures can relax a restrictive selection rule
to allow quasielastic electron coupling with an odd number of flexural phonons, contributing to the increase of the Lorenz ratio towards the Sommerfeld limit at an intermediate temperature sandwiched between the low-temperature hydrodynamic regime and the inelastic electron-phonon scattering regime above 120 kelvin. In contrast to past practices of neglecting the contributions of flexural phonons to transport in two-dimensional materials, this work suggests that tunable electron-flexural phonon couping can provide a handle to control quantum matter at the atomic scale, such as in magic-angle twisted bilayer graphene
where low-energy excitations may mediate Cooper pairing of flat-band electrons
.
The densification of integrated circuits requires thermal management strategies and high thermal conductivity materials
. Recent innovations include the development of materials with thermal ...conduction anisotropy, which can remove hotspots along the fast-axis direction and provide thermal insulation along the slow axis
. However, most artificially engineered thermal conductors have anisotropy ratios much smaller than those seen in naturally anisotropic materials. Here we report extremely anisotropic thermal conductors based on large-area van der Waals thin films with random interlayer rotations, which produce a room-temperature thermal anisotropy ratio close to 900 in MoS
, one of the highest ever reported. This is enabled by the interlayer rotations that impede the through-plane thermal transport, while the long-range intralayer crystallinity maintains high in-plane thermal conductivity. We measure ultralow thermal conductivities in the through-plane direction for MoS
(57 ± 3 mW m
K
) and WS
(41 ± 3 mW m
K
) films, and we quantitatively explain these values using molecular dynamics simulations that reveal one-dimensional glass-like thermal transport. Conversely, the in-plane thermal conductivity in these MoS
films is close to the single-crystal value. Covering nanofabricated gold electrodes with our anisotropic films prevents overheating of the electrodes and blocks heat from reaching the device surface. Our work establishes interlayer rotation in crystalline layered materials as a new degree of freedom for engineering-directed heat transport in solid-state systems.
High-performance thermally insulating materials from renewable resources are needed to improve the energy efficiency of buildings. Traditional fossil-fuel-derived insulation materials such as ...expanded polystyrene and polyurethane have thermal conductivities that are too high for retrofitting or for building new, surface-efficient passive houses. Tailored materials such as aerogels and vacuum insulating panels are fragile and susceptible to perforation. Here, we show that freeze-casting suspensions of cellulose nanofibres, graphene oxide and sepiolite nanorods produces super-insulating, fire-retardant and strong anisotropic foams that perform better than traditional polymer-based insulating materials. The foams are ultralight, show excellent combustion resistance and exhibit a thermal conductivity of 15 mW m(-1) K(-1), which is about half that of expanded polystyrene. At 30 °C and 85% relative humidity, the foams retained more than half of their initial strength. Our results show that nanoscale engineering is a promising strategy for producing foams with excellent properties using cellulose and other renewable nanosized fibrous materials.
I review theoretical ideas and implications of experiments for the gap structure and symmetry of the Fe-based superconductors. Unlike any other class of unconventional superconductors, one has in ...these systems the possibility to tune the interactions by small changes in pressure, doping or disorder. Thus, measurements of order parameter evolution with these parameters should enable a deeper understanding of the underlying interactions. I briefly review the “standard paradigm” for s-wave pairing in these systems, and then focus on developments in the past several years which have challenged this picture. I further discuss the reasons for the apparent close competition between pairing in s- and d-wave channels, particularly in those systems where one type of Fermi surface pocket – hole or electron – is missing. Observation of a transition between s- and d-wave symmetry, possibly via a time reversal symmetry breaking “s + id” state, would provide an important confirmation of these ideas. Several proposals for detecting these novel phases are discussed, including the appearance of order parameter collective modes in Raman and optical conductivities. Transitions between two different types of s-wave states, involving various combinations of signs on Fermi surface pockets, can also proceed through a T-breaking “s + is” state. I discuss recent work that suggests pairing may take place away from the Fermi level over a surprisingly large energy range, as well as the effect of glide plane symmetry of the Fe-based systems on the superconductivity, including various exotic, time and translational invariance breaking pair states that have been proposed. Finally, I address disorder issues, and the various ways systematic introduction of disorder can (and cannot) be used to extract information on gap symmetry and structure.
Je passe en revue les idées théoriques et les implications des expériences sur la structure et la symétrie du gap dans les supraconducteurs à base de fer. Contrairement à la majorité des autres classes de supraconducteurs non conventionnels, il est ici possible de modifier les interactions par de faibles changements de pression, par dopage ou par l'introduction de désordre. Aussi, des mesures de l'évolution du paramètre d'ordre en fonction de ces paramètres de contrôle devraient permettre une compréhension fine des interactions sous-jacentes responsables de l'appariement des électrons. Je rappelle brièvement le « paradigme standard » de la supraconductivité de type s dans ces composés, et discute plus finement les développements effectués ces dernières années qui mettent en cause ce modèle. Je discute les raisons qui semblent conforter une compétition entre des appariements de type s et d, particulièrement dans les systèmes pour lesquels une des poches de la surface de Fermi – électrons ou trous – est absente. L'observation d'une transition entre symétries s et d, éventuellement associée à un état « s + id » brisant la symétrie T par renversement du temps, serait une importante confirmation de ces idées. Plusieurs propositions permettant d'observer ces transitions sont discutées, incluant l'apparition de modes collectifs du paramètre d'ordre dans les expériences d'effet Raman ou de conductivité optique. Des transitions entre différents types d'états de type s impliquant diverses combinaisons de signes sur les poches de la surface de Fermi peuvent aussi se produire à travers un état « s + is » brisant T. Je discute des travaux récents qui suggèrent que l'appariement peut, de façon surprenante, s'effectuer sur une grande gamme d'énergies autour de l'énergie de Fermi, ainsi que de l'effet sur la supraconductivité de la symétrie de plan de glissement miroir des composés au fer et des divers états de paires exotiques brisant l'invariance par renversement du temps ou par translation qui ont été proposés. Finalement, je considère les problèmes associés au désordre et comment diverses façons d'introduire un désordre contrôlé peuvent (ou non) permettre d'obtenir des informations sur la structure et la symétrie du gap.
•Accurate measurements of a broad range of in-plane thermal conductivity from 1-2000 W/(m•K) with a typical uncertainty of <5%.•Simultaneously determining the laser spot size with an uncertainty of ...<2%.•Measurements of sub-millimeter scale samples with a lateral size >0.1 mm.•Measurements of the anisotropic in-plane thermal conductivity tensor even using an elliptical laser spot.
In-plane thermal conductivities of small-scale samples are hard to measure, especially for the lowly conductive ones and those lacking in-plane symmetry (i.e., transversely anisotropic materials). State-of-the-art pump-probe techniques including both the time-domain and the frequency-domain thermoreflectance (TDTR and FDTR) are advantageous in measuring the thermal conductivity of small-scale samples, and various advanced TDTR and FDTR techniques have been developed to measure transversely anisotropic materials. However, the measurable in-plane thermal conductivity (kin) is usually limited to be >10W/(m·K). In this work, a new spatial-domain thermoreflectance (SDTR) method has been developed to measure a broad range of kin of millimeter-scale small samples, including those lacking in-plane symmetry, extending the current limit of the measurable kin to as low as 1 W/(m·K). This SDTR method establishes a new scheme of measurements using the optimized laser spot size and modulation frequency and a new scheme of data processing, enabling measurements of in-plane thermal conductivity tensors of a broad range of kin values with both high accuracy and ease of operation. Some details such as the requirement on the sample geometry, the effect of the transducer layer, and the effect of heat loss are also discussed. As a verification, the kin of some transversely isotropic reference samples with a wide range of kin values including fused silica, sapphire, silicon, and highly ordered pyrolytic graphite (HOPG) have been measured using this new SDTR method. The measured kin agree perfectly well with the literature values with a typical uncertainty of <5%. As a demonstration of the unique capability of this method, the in-plane thermal conductivity tensor of x-cut quartz, an in-plane anisotropic material, has also been measured.
The high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New materials with high thermal conductivity are needed to ...effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report the experimental discovery of high thermal conductivity at room temperature in cubic boron arsenide (BAs) grown through a modified chemical vapor transport technique. The thermal conductivity of BAs, 1000 ± 90 watts per meter per kelvin meter-kelvin, is higher than that of silicon carbide by a factor of 3 and is surpassed only by diamond and the basal-plane value of graphite. This work shows that BAs represents a class of ultrahigh-thermal conductivity materials predicted by a recent theory, and that it may constitute a useful thermal management material for high-power density electronic devices.
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